Description of the Prior Art
It has been proposed to vary the damping applied to wheel carriers in a road vehicle wheel suspension system electrically to obtain desirable displacement/acceleration characteristics as is disclosed in our U.S. Patent Application Ser. No. 500,577 filed Aug. 26, 1974, now U.S. Pat. No. 3,995,883, which is uncorporated herein by reference. In general terms, however, a relatively low power electrical signal should desirably be used and this leads to problems in obtaining sufficient power to operate a damping control valve. A natural choice would be to use an electro-hydraulic servo-mechanism to operate the valve but this would require a source of hydraulic power to operate the valve.